We investigated the ability of human recombinant (hr) hrIL-6 to augment antibody directed cell mediated cytotoxicity (ADCC) via MAb D612 using colorectal carcinoma target LS-174T, WiDr and HT-29 cells. A significant increase in ADCC activity was observed after human PMNC were incubated in 100-400 U/ml of hrIL-6. hrIL-6 did not augment non specific (non MAb mediated) cytotoxicity. Enhancement of ADCC activity was blocked by the addition of an antibody against hrIL-6 but not by an antibody to the IL-2 receptor, suggesting the hrIL-6 augmentation of ADCC activity may not be mediated through IL-2. The surface antigen expression and susceptibility to lysis of human IL-6 gene transfected colorectal carcinoma cell line, HT-29 were investigated by flow cytometry and by 24 h In-Ill release ADCC assay. We found that IL-6 transfected HT-29 cell can secrete a high level of biologically active IL-6. The expression of cell surface antigens in the transfected HT-29 cells were analyzed. Significant enhancement in the percent of CEA expressing cells as detected by COL-1 MAb but not in the percent of cells expressing HLA-class I, HLA class II, and ICAM-1 antigens as compared to the parental HT-29 cell was observed. The susceptibility to lysis of the transfected HT-29 cells increased significantly in ADCC assay using COL-1 MAb. The increase in ADCC activity correlated well with the increased expression of CEA. These results provide a rationale for use of IL-6 gene transfer into human cells as a possible modality for cancer therapy. We also investigated the tumoricidal properties of D612 MAb alone and in combination with IL-2 activated tumor lymphocytes in athymic mice bearing LS-174T colon tumor xenografts. The results demonstrate that the tumoricidal properties of LAK cells and the D612 MAb can be augmented when used together in immunotherapy of human colon cancer xenograft.